Paul,
You didn't mention if this was a new phenomenon on a new design or if its a
continuous problem, anyway you asked for them, so here are my two cents
worth...
If this is a new condition, you must question have CAD model land geometries
been modified recently?
Does the stencil match land geometries. Has a new stencil supplier been
engaged recently? 

Obviously tombstoning is caused from force imbalances during the reflow
soldering process.
It can be caused by:
1) Uneven oven profile/heating causing a differential across the component
terminals. 
2) Uneven paste deposition on the lands.
3) Insufficient tack force of the solder paste to hold the component in
place during reflow. 
4) Excessive movement during and after the reflow operation can cause
component misalignment which could result in tombstoning.
5) Inadequate placement force to make intimate contact between the paste and
the termination of the pads can also be a cause. 

Possible solutions include: 
1) Increasing the preheating temperature (following the recommended
guidelines) so that the temperature differential between the terminations is
low at the time of reflow. 
2) Ensure consistent solder paste deposit heights between pads via a vision
system designed to measure solder paste deposition height. 
3) Avoid inefficient tack force by avoiding extreme environmental
conditions. 
4) Minimizing the amount of movement the assembly sees during reflow. 
6) Increasing component placement force to ensure contact of the component
terminal to the solder paste deposit (not too much because bridging may
occur if it is too high or if very tight component proximities).

Environmental conditions worth questioning:
What is the temperature of the solderpaste?
Too high or low temperature conditions or too high or low relative humidity
can have an effect.
Too hot and the solder paste may slump which can lead to bridging etc.
Too cold and solder paste may become too thick to yield good print
definition
Are you experiencing stencil clogging?
If the solder paste is exposed to extreme high or low humidity, similar
problems will result.
The recommended temperature the solder paste should be at when printing is
70-77°F, the recommended humidity is between 35-65% RH.

These issues were derived from a handout received from Kester Solder company
a few years ago but I'm sure are still valid! 

 Good Luck!
Mike Beckman
ATD DPD Substrate Design Integration
[log in to unmask]
Ofc. 602 554-4232




-----Original Message-----
From: Paul [mailto:[log in to unmask]]
Sent: Monday, May 10, 1999 3:47 PM
To: [log in to unmask]
Subject: [TN] Tombstoning 0402 caps.... Help needed.


Hi All,
        I am looking for any information concerning the reduction of
tombstoning.

        The only parameters that we can adjust are the size of the paste
dot, the type of paste used (any composition as long as it is eutectic
PB/SN), and the reflow profile (two atmosphere choices).

        Since the physical pheomeonon is centered around surface tension I
am very interested to read about any study that was done that resulted in
reduced surface tension.

        Currently the process is in the fractional percentages, the goal
is to go below 100 DPM.

Thanks,
Paul Gill
Process Engineer

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